Instrumentation amplifier with extended common mode range

A differential input instrumentation amplifier circuit capable of operating from a single power supply and having a common mode input voltage range that substantially exceeds the voltage span of said power supply, said circuit comprising:

• first and second power supply rails connectible to a single power supply, said second rail comprising the ground connection for said circuit;
• first and second transistors, each having emitter, base, and collector electrodes;
• first and second current sources coupled respectively between the collectors of said first and second transistors and said first power supply rail;
• means for operating said first and second transistors at substantially equal current densities to establish equal emitter to base voltages;
• first and second feedback amplifier means coupled respectively between the collector and base electrodes of said first and second transistors to provide a negative feedback loop and to adjust the base to collector potentials of each of said transistors to a value that produces collector currents equal to the currents established by said first and second current sources, and wherein said emitter electrodes constituting the differential input terminals of said amplifier and said emitter of said second transistor is connectible to a remote ground connection to said circuit;
• first resistor means coupled between said base electrodes of said first and second transistors;
• second resistor means coupled between said base and emitter electrodes of said second transistor to establish a reference current in said second feedback amplifier means;
• and
• means for responding to the current flowing in said first feedback amplifier to provide said amplifier output, said output being the ratio of the potential difference existing between said first and second transistor emitters divided by the value of said first resistor means and wherein said circuit is operative with said remote ground operating at a potential substantially below that of said circuit ground.